When a fluid is subjected to external forces, it resists flow due to internal friction. Viscosity is a measure of this internal friction.
The viscosity of oils of lubricating viscosity is generally dependent upon temperature. As the temperature of the oil is increased, the viscosity usually decreases, and as the temperature is reduced, the viscosity usually increases.
The function of a viscosity improver is to reduce the extent of the decrease in viscosity as the temperature is raised or to reduce the extent of the increase in viscosity as the temperature is lowered, or both. Thus, a viscosity improver ameliorates the change of viscosity of an oil containing it with changes in temperature. The fluidity characteristics of the oil are improved.
Viscosity improvers are usually polymeric materials and are often referred to as viscosity index improvers.
Ester group containing polymers are well-known additives for improving the fluidity characteristic of lubricating oils. Polyacrylate, particularly polymethacrylate ester polymers are well-known and are widely used for this purpose.
Dispersants are also well-known in the lubricating art. Dispersants are employed in lubricants to keep impurities, particularly those formed during operation of machinery, in suspension rather than allowing them to deposit on the surfaces of lubricated parts.
Multifunctional additives that provide both viscosity improving properties and dispersant properties are likewise known in the art. Such products are described in numerous publications including Dieter Klamann, "Lubricants and Related Products", Verlag Chemie Gmbh (1984), pp. 185-193; C. V. Smalheer and R. K. Smith "Lubricant Additives", Lezius-Hiles Co. (1967); M. W. Ranney, "Lubricant Additives", Noyes Data Corp. (1973), pp. 92-145, M. W. Ranney, "Lubricant Additives, Recent Developments", Noyes Data Corp. (1978), pp. 139-164; and M. W. Ranney, "Synthetic Oils and Additives for Lubricants", Noyes Data Corp. (1980), pp. 96-166. Each of these publications is hereby expressly incorporated herein by reference.
Derivatives of polyacrylate esters are well-known as additives for lubricants that provide not only improved viscosity characteristics, but also may enhance dispersant properties of lubricants.
It is desirable that the viscosity improver or dispersant viscosity improver not adversely affect the low-temperature viscosity of the lubricant containing same. Frequently, while viscosity improvers or dispersant viscosity improvers enhance the high temperature viscosity characteristics of lubricating oil, that is, they reduce the loss of viscosity with increasing temperature, low temperature properties of the treated lubricant become worse. It is also desirable that viscosity improving agents provide relatively constant fluidity characteristics under extended periods of use. Such materials usually are polymers that resist shearing.
While many lubricating oil compositions can benefit from the use of viscosity improvers, gear lubricants and automatic transmission fluids are particularly sensitive to viscosity.
Gear lubricants are subjected to high levels of shearing under use. While many conventional viscosity improving agents provide lubricants with exemplary viscosity characteristics when first prepared, after a short period of use lubricating a gear set, such as in an automobile axle or transmission, the viscosity improving agent undergoes shearing, resulting in reduced viscosity improving properties. Gear lubricants also preferably have good low temperature viscosity characteristics.
One of the major requirements for automatic transmission fluids has been improved low temperature performance. This is demonstrated by requirements for maximum Brookfield viscosities of 50,000, 20,000, 10,000 centipoise, or even lower at -40.degree. C. The viscosity modifier, which can comprise nearly 50 weight percent and sometimes even more of the total additive system employed in an automatic transmission fluid can have a major impact on the low temperature performance. Low temperature characteristics are also desirable in other applications such as in gear lubricants and manual transmission fluids where the viscosity modifier may also comprise substantial amounts of the total additive system.
The copolymers of this invention are also useful in many other lubricating oil compositions including, but not limited to engine oils, hydraulic oils, manual transmission oils, lubricants for continuously variable transmissions (CVT) such as the well-known belt driven version invented by Van Doome, industrial oils, etc.
Accordingly, it is desirable to provide compositions that reduce the extent of loss of viscosity at high temperatures while not adversely increasing the low temperature viscosity of lubricating oil compositions.
It is also desirable to provide viscosity improving agents that resist shearing under conditions of use.
Particularly desirable are viscosity improving agents that both resist shearing and reduce the extent of loss of viscosity at high temperatures while not adversely increasing the low temperature viscosity of lubricating oil compositions.
It is another object of this invention to provide novel multi-purpose lubricant additives.
A more specific object is to provide multi-purpose additives directed to improving the viscosity and dispersant properties of a lubricating composition.
A further object is to provide processes for preparing copolymers that improve viscosity characteristics of lubricants over a wide range of temperature conditions.
Still another object is to provide additive concentrates containing the novel copolymers of this invention.
Yet another object is to provide lubricants having improved dispersant and viscosity properties.
Other objects will in part be obvious in view of this disclosure and will in part appear hereinafter.